1. Technical Field
The invention relates to a plain bearing shell having a slide face which is partially convexly curved. The invention also relates to a method for producing a plain bearing shell of this type.
2. Related Art
The machining of the slide faces of bearing shells is carried out by drilling out the bearing shells. This involves a drill spindle rotating at a rotation speed X (rev/min) and moving at a feed rate Y (mm/rev) in the axial direction of the bearing shell to be machined. The drilling out is carried out, for example, using a drill spindle with two cutting cartridges positioned opposite one another at 180° being installed in the spindle head thereof.
The drill spindle comprises a first cutting cartridge for cutting a bearing shell to a certain wall thickness (continuously or non-continuously) and, optionally, a second cutting cartridge for forming so-called exposed regions. An exposed region is to be understood as a region at the ends of the bearing shell or in the region of the partial surfaces, in which the wall thickness of the bearing shell is reduced as compared to the wall thickness of the rest of the bearing shell. In this way, it is possible to reduce the wear of a shaft running in the bearing shell, which is due to inaccuracies at the joins of the two bearing shells forming a bearing. The two cutting cartridges positioned opposite one another at 180° are arranged so as to be axially displaced on the spindle head. It is possible to adjust the diameter of the cutting circle when the drill spindle is idle.
During the machining process, the two cutting cartridges are fixed in position with respect to their axial direction, i.e. in the radial direction of the drill spindle.
As a result of the precisely linear feed movement of the drill spindle, a slide face surface geometry also machined in a precisely linear manner is created on the plain bearing shell.
Such a bearing shell 1′ is shown in FIG. 1. The profile-free slide face is designated with 2′ and the two partial surfaces at the edge regions of the bearing shell 1′ are designated with 3′. A cross section through the conventional profile-free bearing shell (along the dashed and dotted line shown in FIG. 1) is shown in FIG. 2. The slide face 2′ is flat along the axial direction of the bearing shell.
Particularly in combustion engines, very high loads occur until the contact between the outer regions of the plain bearing shell, viewed in the axial direction of the plain bearing shell, and the shaft. High oil pressures between the outer regions of the plain bearing shell, in the axial direction of the plain bearing shell, and the shaft lead to a great deal of oil loss at the plain bearing position during operation. The tearing of the oil film between the plain bearing shell and the shaft results in an extremely high level of wear and material fatigue in the region of the contact zone between the plain bearing shell and the shaft. One cause for the wear of the slide face at the bearing and the shaft contained therein is a minimal bending or tilting of the shaft under a load, as shown schematically in FIG. 4 with a greatly exaggerated tilting of the shaft W. If the bearing is a connecting rod bearing or a main bearing in a combustion engine that is subject to high loads, these uneven loads of the shaft on the bearing ultimately have an adverse effect on the operating performance of the engine. In particular, serious deficiencies having a considerable effect on the operating state of an engine may occur particularly in the regions of the plain bearing shell that are subject to the highest loads, the apex of the plain bearing shell. The deficiencies caused by this may lead to disruptions to the operating state of the engine or even to a complete breakdown of the engine.
According to DE 102 08 118 A1, this problem is conventionally solved by configuring the two end portions of the plain bearing (viewed in the axial direction of the bearing) to be round, as shown in FIGS. 3 and 5. The curvature at the edge regions 21′ prevents the end portions of the plain bearing from striking against each other when the crank pin or the shaft bends under a load.
According to DE 102 08 118 A1, in addition to the problem of the shaft bending under a load, the problem of the deformation of the bearing under a load must also be taken into consideration. In the middle portion of the plain bearing, viewed in the axial direction, there is a higher oil pressure than in the edge regions. The location-dependent oil pressure causes the slide face of the bearing to be concavely deformed such that even if the slide face is configured to be round at the edge regions, the oil film may tear. However, a greater degree of roundness is not consistent with maximizing the load-bearing region.
To solve this problem, which is to be distinguished from the tilting or bending of the shaft, DE 102 08 118 A1 proposes a plain bearing comprising a cylindrical bearing surface and an inclined end portion on each axial end face of the bearing surface. The inclined surface extends from a certain axial position of the bearing surface to the edge, viewed in the axial direction of the bearing, with the axial length of the inclined surface along the circumference of the cylindrical bearing surface varying. The axial length of the inclined surface in the circumferential direction of the bearing is adapted to the deformations which may occur under the load of the sliding layer. In one embodiment, the axial length of the inclined surface is selected such that it is longest at the apex of the bearing shell and shorter toward the circumferential ends of the bearing shell. By choosing the degree of the slant and the axial length of the slant at the end regions, compensation for the deformation of the bearing shell owing to the oil pressure is sought.
To be distinguished from this is the problem mentioned above of the bending or tilting of the shaft in the bearing, which is conventionally dealt with by rounding off the ends of the bearing shell over the entire circumference of the bearing. A slight rounding off of the ends of the bearing shell does not sufficiently solve the problem of the oil film tearing owing to the bending or tilting of the shaft. However, a very round configuration of the slide face reduces the guiding accuracy of the shaft in the bearing.